A process oil is typically added to a rubber material used for manufacturing a tire in order to enhance mechanical performances and processability of the rubber material. The process oil is used as: a constituent of a plasticizer for a thermoplastic resin; a constituent of a printing ink; and a lubricant or a solvent component used for a softener or the like for a recycled asphalt, as well as being used with rubber materials such as a natural rubber and a synthetic rubber.
For a process oil used as an additive for the rubber materials, there has been conventionally employed an extract that is a by-product produced in manufacturing a paraffinic base oil by solvent refining. However, due to a carcinogenicity problem, noncarcinogenic aromatic oils for tires have been being manufactured by various manufacturing methods.
In manufacturing a rubber for a tire, since compatibility of the process oil and the rubber material is regarded as important, an aromatic component of the process oil is an important factor. However, since excessive dependency on the aromatic component leads to increase of the carcinogenicity of the process oil, the aromatic oil as a source of the carcinogenicity has to be removed while maintaining the compatibility of the process oil and the rubber material in a proper state.
Furthermore, a hazardous effect of polycyclic aromatics (PCA; same meaning as PAH (PolyAromatic Hydrocarbon)) contained in the process oil has been acknowledged as a problem in recent years. Especially, since the process oil used for an automobile tire pollutes the environment via dusts of the tire, there has been a demand for reducing the PCA contained in the process oil. In Europe or other areas, a mineral oil having a PCA content of 3 mass % or more is restricted in handling. However, since an extracted oil with high aromatics content obtained by conventional manufacturing methods contains high contents of the polycyclic aromatics, it has been urgently required to develop a process oil with reduced PCA (specifically, less than 3 mass %) and a manufacturing method thereof.
With such background, there has been studied a technology for manufacturing a process oil with reduced PCA content and capable of balancing the compatibility with a rubber material and non-carcinogenicity. For example, a technology related to a manufacturing method of a process oil for a tire rubber has been disclosed, in which a naphthenic asphaltene and a solvent-extracted oil are combined (see, for instance, Patent Document 1). As another technology, there has been disclosed a manufacturing method of a process oil using an extract obtained by solvent-extracting a deasphalted oil (see, for instance, Patent Document 2 and Patent Document 3).    [Patent Document 1] JP-A-11-80434    [Patent Document 2] JP-A-2000-80208    [Patent Document 3] JP-A-2002-3861